As typically seen in a liquid crystal display device, a thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but can be formed over a larger glass substrate. In contrast, a thin film transistor manufactured using crystalline silicon has high field effect mobility, but due to a crystallization step such as laser annealing, such a transistor is not always suitable for being formed over a larger glass substrate.
Techniques have attracted attention, in which a thin film transistor using an oxide semiconductor is manufactured and applied to electronic devices or optical devices. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O based oxide semiconductor for an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.